The present invention pertains to microlithography apparatus and methods using a charged particle beam (e.g., electron beam or ion beam) as an energy beam for performing transfer-exposure of a pattern from a mask or reticle to a sensitive substrate. Such methods and apparatus are used, for example, in the manufacture of semiconductor integrated circuits and displays. More specifically, the invention pertains to alignment and fiducial marks used in such methods and apparatus, wherein a first such mark is situated as an index mark on a first surface (e.g., reticle surface) and a second such mark is situated as a fiducial (reference) mark on a second surface (e.g., sensitive substrate such as a wafer or the like), and an alignment of the two marks is performed to achieve optimal positioning of the two marks relative to each other.
One type of conventional charged-particle-beam (CPB) microlithography (projection-exposure) apparatus utilizes an electron beam to irradiate a pattern defined on a reticle. Electrons of the beam passing through the irradiated region of the reticle are projected and focused onto a sensitized substrate (e.g., semiconductor wafer), thereby xe2x80x9ctransferringxe2x80x9d the pattern from the reticle to the wafer. The reticle is mounted on a reticle stage and the wafer is mounted on a wafer stage. For accurate projection-exposure of the pattern from the reticle to a particular region on the wafer, it is necessary to align accurately the wafer with the substrate. To such end, at least one xe2x80x9calignment markxe2x80x9d (also termed an xe2x80x9cindex markxe2x80x9d) is provided on the reticle or reticle stage, and at least one xe2x80x9cfiducialxe2x80x9d (reference) mark is provided on the wafer or wafer stage. In a procedure for aligning the wafer with the reticle, the index mark is aligned with the fiducial mark.
More specifically, in a representative conventional method for performing alignments as summarized above, the electron beam is caused to illuminate the index mark on the reticle or reticle stage. Electrons of the beam passing through the index mark are directed as a scanning beam to the fiducial mark on the wafer or wafer stage. Electrons of the beam that are backscattered from the fiducial mark are detected using an appropriate detector, and the relative positional relationship of the projected image of the index mark to the fiducial mark is determined. Based on the determination, the reticle and wafer are aligned as required. Such determinations also can provide data on distortion of the beam, preliminary to making appropriate corrections to the beam.
To obtain accurate determinations as summarized above, it is necessary that the location of the index mark be known accurately relative to, for example, detected positional coordinates of the reticle stage. Similarly, it is necessary that the location of the fiducial mark be accurately known relative to the detected positional coordinates of the wafer stage. Therefore, and in view of the fact that the marks are irradiated by the beam, it is important that the marks be defined on a material having as low a coefficient of thermal expansion as possible so as to undergo minimal thermal deformation when irradiated.
FIG. 1 is a schematic sectional view of a conventional fiducial mark 10 body. A fiducial mark typically is produced by forming a layer of a heavy metal such as Ta or W on the surface of a substrate 11 (made of Si or other suitable material). The elements 12 of the fiducial mark are formed by etching the heavy-metal layer appropriately.
The fiducial mark body 10 of FIG. 1 normally is situated on the wafer or wafer stage and used in conjunction with a corresponding index mark on the reticle or reticle stage. The pattern of the index mark normally is similar to the pattern of the fiducial mark 10. An electron beam irradiates the index mark such that an image of the alignment mark is formed on or near the fiducial mark 10. As the electron beam scans the image of the index mark over the fiducial mark, the relative positions of the marks are determined from an electrical signal produced by a detector of electrons that are backscattered (xe2x80x9cbackscattered electronsxe2x80x9d or BSEs) from the fiducial mark. Based on the signal, an upstream deflector can be energized appropriately to deflect the beam to achieve maximal coincidence of the marks.
In charged-particle-beam (CPB) microlithography, alignment of an index mark with a corresponding fiducial mark can be performed using either an optical-based alignment sensor (i.e., a sensor utilizing light) or a CPB-based alignment sensor (i.e., a sensor sensitive to charged particles such as BSEs from the beam). Especially whenever an optical-based alignment sensor is used, it is necessary to determine, as a calibrated xe2x80x9cbaseline,xe2x80x9d the distance between a reference (fiducial) location of the CPB-optical system and a reference (fiducial) location of the optical-based alignment sensor. Since the optical-based alignment sensor is situated usually outside the xe2x80x9ccolumnxe2x80x9d (vacuum housing) of the CPB-optical system, the distance typically is substantial.
Measurements of distances between fiducial locations can be affected adversely by apparatus vibrations. To eliminate such vibrations, it is necessary to measure simultaneously the reference location of the CPB-optical system and the reference location of the optical-based alignment sensor. The fiducial mark used in baseline measurements should have a length that is at least equal to the baseline length. That is, when an optical-based alignment sensor is used, it is important to measure a xe2x80x9cbaselinexe2x80x9d (distance between the optical axis of the charged particle beam and the optical axis of the optical system of the alignment sensor). To such end, fiducial marks are used that can be measured at the same time. Such marks should extend over the baseline or, if spaced apart from each other at the baseline, overlap each other. However, such marks can be adversely affected easily.
Also, positional stability of the fiducial marks to changes in temperature is very important. By way of example, if the distance between the reference location of the CPB-optical system and the reference location of the optical-based alignment sensor is 20 mm, and the apparatus temperature is controlled to within xc2x10.5xc2x0 C., the coefficient of thermal expansion of the substrate on which the fiducial mark is formed should be 1xc3x9710xe2x88x927/xc2x0 C. or less to suppress variations in measured distance between the respective fiducial marks adequately to within 1 nm or less. The coefficient of thermal expansion of Si as currently used as a substrate for fiducial marks is about 2.4xc3x9710xe2x88x925/xc2x0 C., which is unsatisfactorily high for use in obtaining accurate measurement of the distance between fiducial locations.
It has been proposed to manufacture the substrate for a fiducial mark body using a substance having a low coefficient of thermal expansion, such as ZERODUR made by Schott of Germany. However, because ZERODUR is not electrically conductive, an undesirable electrical charge tends to accumulate on it whenever it is irradiated by an electron beam. The accumulated charge forms a corresponding electrical field around the fiducial mark, which can perturb the beam incident on the mark. If substrate charging is excessive, an electrical discharge may occur which can destroy the fiducial mark.
Providing ZERODUR with a conductive metal coating has been proposed to prevent or at least reduce charge accumulation on the substrate. However, such a coating tends to reduce the contrast of the BSE signal.
Therefore, methods are required for preventing charge accumulation on a fiducial mark without adversely reducing contrast and while maintaining the low thermal-expansion characteristic of the fiducial-mark substrate.
In view of the shortcomings of the prior art as summarized above, the present invention provides, inter alia, fiducial mark bodies that can be used for any of various applications in charged-particle-beam (CPB) microlithography. For example, fiducial mark bodies according to the invention can be used for measuring the distance from a reference position of an electron-optical system and a reference position of an optical-based alignment sensor, as used in a CPB microlithography apparatus.
A first exemplary embodiment of a fiducial mark body is especially suitable for mounting on a wafer stage of a CPB microlithography apparatus. The mark body, defining a fiducial mark, can be used, e.g., for apparatus calibration, reticle alignment with the wafer, etc. Elements (features) of the fiducial marks are defined by respective portions of a layer of heavy metal formed on a substrate plate. The substrate plate is formed of a material having a coefficient of thermal expansion of 10xe2x88x927/xc2x0 C. or less. The substrate plate is coated with a film of an electrically conductive material (other than the heavy metal). By forming the substrate plate of a material having a coefficient of thermal expansion that is 10xe2x88x927/xc2x0 C. or less, measurement errors arising from changes in temperature of the fiducial mark body are reduced to 1 nm or less whenever the fiducial mark is being used for measuring the distance between a reference position of the CPB-optical system of the CPB microlithography apparatus and a reference position of an optical-based alignment sensor. Exemplary materials for the substrate plate include quartz and glass-ceramics such as ZERODUR(trademark).
Materials having a low coefficient of thermal expansion generally are not electrically conductive. As a result, such materials can become charged electrically whenever they are irradiated by a charged particle beam. Such charge accumulation adversely can affect electrons backscattered from a fiducial mark made of such material, thereby adversely affecting the results of measurements performed using such marks. Hence, in this embodiment, the surface of the substrate plate, (other than where the mark elements are formed of heavy metal) is covered with a layer of an electrically conductive xe2x80x9clight metal.xe2x80x9d The light-metal covering prevents charging of the fiducial mark body and allows greater accuracy of measurements performed using the mark body. Also, the light-metal covering exhibits very low production of secondary electrons or backscattered electrons that otherwise would affect measurement results adversely.
Especially desirable xe2x80x9cheavy metalxe2x80x9d materials for use in defining the mark elements are Ta, W, and Pt. Especially desirable xe2x80x9clight metalxe2x80x9d materials for use in forming the electrically conductive layer are Ti, Cr, and Al.
The thickness of the light metal layer can be 1 xcexcm or less. The coefficient of backscattered electrons exhibited by a substance (the coefficient expressing the relative generation of backscattered electrons when the substance is irradiated by a charged particle beam) increases with increased thickness of the substance. When the thickness of the substance reaches a specific thickness for the particular substance, the coefficient reaches a critical value. For the stated light metals, the critical value is reached whenever the thickness is greater than 1 xcexcm. Hence, by keeping the layer of light metal less than 1 xcexcm, the coefficient of backscattered electrons is maintained acceptably small so as to produce an acceptably small number of backscattered electrons to avoid adverse effects on measurement accuracy.
A second exemplary embodiment of a fiducial mark body according to the invention is also especially suitable for mounting on the wafer stage. The mark body comprises a substrate plate as summarized above. The mark elements are defined by a layer of heavy metal as summarized above. The substrate plate and mark elements are coated with a layer of an electrically conductive xe2x80x9clight metalxe2x80x9d as summarized above. As the first exemplary embodiment, the fiducial mark body of the second embodiment is resistant to charging. A charged particle beam incident on the fiducial mark body of this embodiment passes readily through the layer of xe2x80x9clight metalxe2x80x9d to the mark elements. The layer of xe2x80x9clight metalxe2x80x9d exhibits very little absorption or scattering of the incident beam so as to produce substantially no adverse effects on measurements performed using the mark.
In a third representative embodiment, the mark elements are defined by respective xe2x80x9cthickxe2x80x9d regions of heavy metal, and spaces between and outside the mark elements are formed by relatively xe2x80x9cthinxe2x80x9d regions of heavy metal. Candidate heavy metals are as summarized above. As discussed above with respect to the xe2x80x9clight metal,xe2x80x9d xe2x80x9cheavy metalxe2x80x9d substances also have respective coefficients of backscattered electrons that increase with increased thickness of the substance. Heavy metals also have respective thicknesses above which production of backscattered electrons exhibits a critical increase. Hence, regions of heavy metal other than the actual mark elements are formed of the heavy metal but at a thickness that is thinner than the critical thickness for the material. The regions of heavy metal forming the mark elements are thicker than the critical thickness. As a result, whenever the mark is irradiated using a charged particle beam, much more production of backscattered electrons is exhibited by the mark elements than by regions between or outside the elements.
By way of example, the mark elements can have a thickness (of heavy metal) of 0.5 xcexcm or greater, and the other regions of heavy metal can have a thickness of 0.2 xcexcm or less. In general, the candidate heavy metals generally have a critical thickness ranging from 0.2 to 0.5 xcexcm. If the thicker portions of the heavy-metal layer are greater than about 5 xcexcm thick, generation of backscattered electrons reaches a xe2x80x9csaturationxe2x80x9d level and further increases in thickness exhibit no further differential effect. Hence, the maximum thickness of heavy metal (in regions defining the mark elements) is desirably 5 xcexcm or less.
In a fourth representative embodiment, a layer of heavy metal (used to define mark elements) is formed on a substrate plate made of a material as summarized above. No heavy metal is present in regions outside the actual mark elements. The entire substrate plate and mark elements are coated with a thin film of an electrically conductive xe2x80x9clight metal.xe2x80x9d
In another representative embodiment, the subject fiducial mark body comprises a substrate plate made of a material having a low coefficient of thermal expansion as summarized above. Mark elements are formed on a surface of the substrate plate of heavy metal. A layer of an electrically conductive material (desirably a xe2x80x9clight metalxe2x80x9d as summarized above) is provided within the thickness dimension of the substrate plate or on a rear surface of the substrate plate. The layer of electrically conductive material is connected to ground during use of the fiducial mark body. Because the layer of electrically conductive material does not coat the mark elements in this embodiment, any possible effect of the electrically conductive material on mark contrast essentially is eliminated. The fiducial mark body of this embodiment is especially suitable for attachment to the wafer stage of a CPB microlithography apparatus and is resistant to charging.
In this embodiment, the layer of electrically conductive material desirably is configured so as to have minimal effect on the thermal expansion characteristic of the fiducial mark body while preventing charging of the mark body. To such end, for example, the layer of electrically conductive material can be configured as a network. Alternatively, the electrically conductive material can be layered only in regions of the mark body that are irradiated directly by the charged particle beam (not blocked by the mark elements).
According to another aspect of the invention, methods are provided for producing fiducial mark bodies for use in a CPB microlithography apparatus. The fiducial mark bodies produced by the methods are especially suitable for mounting on the wafer stage of the CPB microlithography apparatus, for use in apparatus calibration, reticle alignments, and other uses.
In a representative embodiment of the methods, a substrate plate is coated with a thin film of an electrically conductive xe2x80x9clight metal.xe2x80x9d The substrate plate and light metal are as summarized above. Mark elements are defined by applying a layer of xe2x80x9cheavy metalxe2x80x9d (as summarized above) and removing portions of the heavy metal not intended to define a mark element.
According to yet another aspect of the invention, CPB microlithography apparatus are provided that comprise a CPB-optical system, an optical-based alignment sensor, and any of the various fiducial mark bodies according to the invention (representative embodiments of which are summarized above). With such an apparatus, the distance from a reference position of the CPB-optical system and a reference position of the optical-based alignment sensor can be measured accurately, thereby facilitating accurate alignments performed using the optical-based alignment sensor.